1. Field of the Invention
This invention relates in general to ladders and in particular to a pole having retractable rungs.
2. Description of the Prior Art
Burglaries and vandalism are constant and costly occurrences to elevated property. One type of property of this nature is ground navigational equipment for aircraft. The navigational equipment is often mounted on towers or poles in remote areas where security systems can not provide ample protection. Even though a particular installation may be fenced, fences can be scaled and fixed ladders are frequently attached to the towers.
Also, technicians from time to time have to spend several hours at the top of the tower testing equipment. If unfenced or in a neighborhood where children are present, there is a danger that a child may climb the ladder even though the technician is at the top.
In other areas, such as defense and atomic energy plants, different security areas may be present on different vertical levels. Access from one vertical level to the other level may need to be secure. A ladder with retractable rungs would be beneficial, particularly one in which the rungs may be retracted both from the top and from the bottom. Also, the retraction mechanism should be capable of being locked, and the lock should be located so that it cannot be broken off or easily cut.
Ladders with hollow housings and retractable rungs are shown in U.S. Pat. Nos. 928,478 and 1,313,444. The device in the first patent utilizes a rack and pinion mechanism at the base for operating and closing the rungs. The device in the latter patent utilizes a lever arrangement at the base. Neither can be manipulated from the top, and both are more complex than desirable. Neither disclose locking mechanisms other than the access door, which unless of a very heavy construction, can be broken open without much effort.